Page 61 - Fundamentals of Air Pollution 3E

III. Regional 37

programs presents formidable tasks. Atmospheric chemistry is discussed
in Chapter 12.

III. REGIONAL

At least three types of problems contribute to air pollution problems on
the regional scale. One is the carryover of urban oxidant problems to the
regional scale. With the existence of major metropolitan areas in close
proximity, the air from one metropolitan area, containing both secondary
pollutants formed through reactions and primary pollutants, flows on to
the adjacent metropolitan area. The pollutants from the second area are
then added on top of the "background" from the first.
A second type of problem is the release of relatively slow-reacting primary
air pollutants that undergo reactions and transformation during long trans-
port times. These long transport times then result in transport distances
over regional scales. The gas, sulfur dioxide, released primarily through
combustion of fossil fuels, both coal and oil, is oxidized during long-distance
transport to sulfur trioxide, SO 3 , which reacts with water vapor to form
sulfuric acid, which reacts with numerous compounds to form sulfates.
These are fine (submicrometer) particulates.
Nitric oxide, NO, results from high-temperature combustion, both in
stationary sources such as power plants or industrial plants in the produc-
tion of process heat and in internal combustion engines in vehicles. The
NO is oxidized in the atmosphere, usually rather slowly, or more rapidly
if there is ozone present, to nitrogen dioxide, NO 2. NO 2 also reacts further
with other constituents, forming nitrates, which is also in fine particulate
form.
Both the sulfates and nitrates existing in the atmosphere as fine particu-
lates, generally in the size range less than 1 /am, can then be removed from
the atmosphere by several processes. One of these is the formation of
clouds, with the particles serving as additional condensation nuclei. These
are then deposited on the ground if the droplets grow to sufficient size to
fall as raindrops (rainout). The other mechanism also involves rain, but is
capture of the particles in air by raindrops falling through the air (washout).
Both of these mechanisms contribute to "acid rain/' which results in the
sulfate and nitrate particles reaching lakes and streams and increasing their
acidity.
A third type of regional problem is that of visibility. Visibility may be
reduced by specific plumes or by the regional levels of particulate matter
that produce various intensities of haze. The fine sulfate and nitrate particu-
lates just discussed are largely responsible for reduction of visibility (see
Chapter 10). This problem is of concern in locations of natural beauty,
where it is desirable to keep scenic vistas as free of obstructions to the view
as possible.